In order to reduce the weight of an automobile, machine parts made of an aluminum alloy containing Si are increasingly employed in place of ferrous machine parts in recent years. In such an Al-Si alloy, the abrasion resistance is extremely improved while the thermal expansion coefficient is reduced and the rigidity is improved, due to the addition of Si. In more concrete terms, an abrasion-resistant aluminum alloy AC8A, an abrasion-resistant cast/forged alloy A390, and an Al-Si alloy obtained by powder metallurgy (PM) have been prepared and put into practice.
However, the AC8A alloy is inferior in abrasion resistance due to a comparatively small content of Si, although the alloy has excellent heat resistance and strength. The A390 alloy on the other hand, is excellent in abrasion resistance with an Si content of 17 to 18 percent by weight, but pro-eutectic Si particles are increased to about 80 .mu.m in size, and hence the cuttability of the alloy is deteriorated to easily cause premature abrasion of a cutting tool. Furthermore, the A390 alloy has inferior plastic workability and toughness. When the A390 alloy is subjected to plastic working such as extrusion or forging after casting, in order to improve the alloy structure for overcoming the aforementioned disadvantages, the coarse Si particles themselves are broken and defects such as pores and voids are caused in interfaces between the Si particles and a matrix material due to their different deformabilities, possibly leading to a deterioration in strength.
In order to compensate for such disadvantages of the A390 alloy, a small amount of an element such as phosphorus (P) may be added to the alloy for suppressing the growth of pro-eutectic Si particles, or the cooling rate during casting may be increased to reduce pro-eutectic Si particle diameters. However, reduction of the Si particles by adding P or the like can be achieved only to a limited extent while increasing the cooling rate for the cast alloy is problematic particularly in view of limitations of the equipment for preparing the same and restrictions depending on the shape and dimensions of the product.
In the Al-Si alloy prepared by powder metallurgy, on the other hand, it is possible to extremely improve heat resistance and abrasion resistance. This is because the alloy tolerance is increased by using rapidly solidified powder which is prepared by air atomization or the like. This allows the introduction of a large amount of Si and addition of a transition metal element such as Fe, Ni, Mn, Cu or Mg and achieves an extreme reduction of pro-eutectic Si particles as compared with a fusion casting method. However, the powder metallurgy method, which requires a high-priced powder raw material and complicated preparation steps, is economically disadvantageous as compared with the fusion casting method.